The present invention relates to a control system for a traction drive transmission including a forward reverse change-over mechanism followed by a traction drive system.
Copending U.S. patent application Ser. No. 07/448,194 now U.S. Pat. No. 5,052,236 and the corresponding European Patent Application No. 89 123 150.8 disclose a traction drive transmission including a change-over mechanism followed by a half-toroidal dual-cavity drive system. This transmission includes hydraulic actuators or servo motors for continuously varying a reduction ratio by varying the angle of inclination of friction or power rollers. A hydraulic control system for the servo motors comprises a forward shift control valve operatively connected to a forward stepper motor, a reverse shift control valve operatively connected to a reverse stepper motor, and a forward reverse change-over valve operatively associated with a collar of a forward/reverse rotational direction detecting mechanism coupled with a counter shaft of the traction drive system. In the forward running, the forward reverse change-over valve assumes a forward position wherein the servo motors are fluidly connected to the forward shift control valve, while, in the reverse running, the forward reverse change-over valve assumes a reverse position wherein the servo motors are fluidly connected to the reverse shift control valve. Each of the shift control valves is drivingly connected to the associated stepper motor via a drive mechanism employing a feed screw. Since the feed screw is used in the drive mechanism, each of the stepper motors is required to produce a relatively large torque, a stepper motor of relatively large type is needed. Besides, two such stepper motors are needed. Thus, it is difficult to reduce the size of the control valve assembly.
An object of the present invention is to improve a hydraulic control system for a traction drive system including a forward reverse change-over mechanism followed by a traction drive system such that the size of the control valve assembly is reduced considerably.